1. Field of the Invention
The present invention refers to a roll train and to a corresponding rolling process with an improved yield.
2. Discussion of the Background
In the rolling process field and in particular in the roll train field, there are known and proved processes and related stands used for the realisation thereof.
To this purpose, in order to realize the rolling step in the blanking phase, the mostly used type of stands is with three stacked rolls or cylinders, the so called "three-high" stand, the rolls being positioned horizontally and having unidirectional drive.
The rolling process is conducted by alternating the passes: a first pass is executed in one direction through the pair of rolls formed by the upper and the middle roll and a second pass is executed in the opposite direction through the pair of rolls formed by the lower and the middle roll. In order to execute such a process, the presence of an operator is generally needed; further, as well known, the productivity is quite low and the temperature of the billet undergoing the rolling process will be subjected to an excessive decrease. Further, a rolling process might be realized using a series of stands having two horizontal rolls, the so called "two-high" stand, positioned in sequence, with the rolls being positioned horizontally and having a unidirectional drive.
The stands positioned in this manner usually generate a torsional stress in the material undergoing the rolling process. This fact, especially in the rolling process of billets having significant dimensions and having the need for expensive toolings, usually causes material surface defects, due just to the torsional stress.
An attempt to solve such problem consists in alternating, in the rolling process, a so called "horizontal" two rolls stand with a so called "vertical" two rolls stand. In other words, these stands have, respectively, horizontal and vertical rolls and a unidirectional drive.
If, from one hand, the arrangement solves the above mentioned problem, on the other hand, it has limitations (especially in the case of special types of steel) on the reduction of section during the various phases of the rolling process, as for the circular/oval, square/diamond sections.
The problem occurs because of the significant material widening under the rolls.
It has to be understood that this widening, which can reach significant values (up to 30-40%), effects significantly the elongation, which, as known, is the true scope of the hot rolling. In order to solve these problems strictly related to the specific process, the section reduction, during the various passes under the various rolls in the realization sequence, has to be dimensionally limited and it is anyhow necessary, as said before, to be present in the more expensive vertical stands.
A blanking type process providing two rolls stands with horizontal rolls, but with a bidirectional drive is also known.
In this case, the rolling sequence is conducted by alternating the passes having opposite rotations or the drive motor.
In this specific case, it is then necessary to have a device for moving, in relation to the various channels, the material undergoing the rolling process and a device for turning the material around its longitudinal axis.
In this manner, the process has almost the same disadvantages as the "three-high" stand.
The same technical problems and disadvantages in the rolling process are found in an intermediate roll train, as presently known.
According to the oldest and most used process, a roll train is provided with stands having two opposed rolls with coplanar axes.
When all the stands are horizontal, a torsional stress is created between the stands, causing surface defects and a significant widening of the material under the rolls. As a consequence, there is a limitation in the reduction of the section during rolling passes. The stands can be alternated according to a horizontal/vertical sequence to avoid the torsional stress; such arrangement will need the use of expensive vertical stands.
In the intermediate rolling process the three high stands, with coplanar roll axes and with an angle displacement of 120.degree. therebetween, are also of common use.
In this case, there are several stands in sequence with three motor driven rolls and with an angle displacement of 60.degree. between two stands in sequence. Using the arrangement, even though a squeezing along three concentric directions is obtained, there is the need to realize a complex drive group of the three rolls which have to be simultaneously activated. This fact greatly increases the costs and the adjustment complexity to obtain proper working conditions. In summary, the known rolling processes with "two-high" stands or alternatively with "three-high" stands, present several significant technical problems in accordance to the various realisations previously examined in details.
In the two-high stands there might be problems related to the vertical stand cost, to the devices needed to move the material, to the need of operating personnel, to the low productivity and to the excessive temperature decrease.
In the three-high stands there are mainly problems related to the expensive mechanical structure to simultaneously drive the three rolls.